A technique for generating combined image data that represents a plurality of images having been joined together is well known in the art. For example, an image processing device reads an original in four reading operations to generate data for four partial images. The image processing device joins the partial image data and outputs data for a single combined image. The image processing device sets the layout position for each partial image in the combined image and determines the edges of the partial images to be joined on the basis of detection results of image densities in edge parts of the partial images.